A rubber coated stainless steel plate obtainable by providing a rubber layer on a stainless steel plate is generally used for a metal gasket, particularly for a head gasket, to be mounted on a vehicle engine. Since the rubber layer of such head gasket is heated to about 200° C. under a high surface pressure while the engine is running, a solid rubber layer that is not foamed (hereinafter referred to as “unfoamed rubber layer”) that hardly deteriorates by heat and pressure is usually used as the rubber layer (see Japanese Patent Application Publication 4-45548(B2), for example).
The metal gasket using the unfoamed rubber layer achieves an excellent sealing property when used together with a bead. Although the metal gasket exhibits the excellent sealing property when a surface roughness of sealed members, e.g., a flange, is 12.5 Ra or less, the sealing property is not ensured in some cases when the surface roughness is higher than 12.5 Ra. Further, since the metal gasket is sealed by concentrating a bolt load on a bead portion (embossed portion), a sufficient sealing property is not achieved in some cases when there is a casting hole (a slight depression on the surface of sealed members, e.g., a flange, caused by foaming during casting; see FIG. 1) across a bead line.
Also, a metal gasket having a foamed rubber layer provided thereon is used (see Japanese Patent Application Publication 5-48742(B2), for example). Although the metal gasket with the foamed rubber layer is capable of sealing a flange having a high surface roughness and a casting hole, it is generally difficult to achieve foaming (particularly with a foaming factor of 2 or more) when a thickness of a rubber layer before foaming is not lower than about 70 μm. Further, the rubber layer before foaming of the metal gasket with the foamed rubber layer is relatively thick, the foamed rubber layer deteriorates under the conditions of a high temperature and a high pressure to result in a reduction in axial force of a bolt. In order to obtain the foamed rubber layer, a micro-encapsulation and a heat decomposition method using a chemical foaming agent are generally employed. Since a foamed rubber layer obtained by the micro-encapsulation has a low foaming factor, such foamed rubber layer has a lower effect for sealing casting holes. Also, since many of cells of the foamed rubber layer are separately closed (hereinafter referred to as “closed cells”), shrinkage of each of the cells is caused at a low temperature (lower than 0° C.), thereby causing a reduction in axial force of a bolt. Further, when a sealing surface pressure is insufficient, the foamed rubber layer does not perfectly crush to cause the foamed rubber layer to deteriorate during use, thereby undesirably resulting in a stress relaxation of a bolt.
As described above, though the gasket with a unfoamed rubber layer is free from a deterioration of the rubber layer under the conditions of a high temperature and a high pressure, such gasket is not useful for sealing a sealed member, e.g., a member having casting holes or the flange having a high surface roughness. In turn, the gasket with the foamed rubber layer has a problem of deterioration under a high temperature and a high pressure.
An object of the present invention is to provide a gasket material to be used for a gasket which is free from a deterioration of a rubber layer under the conditions of a high temperature and a high pressure as well as from a shrinkage of the rubber layer at a low temperature and capable of satisfactorily sealing a sealed member, e.g., a member having casting holes or a flange having a high surface roughness.